1. Field of the Invention
The present invention relates to a semiconductor device having normally-off characteristics, and in particular, relates to a semiconductor device having resistance to a high voltage.
2. Description of the Related Art
A field-effect transistor device such as a high electron mobility transistor (HEMT) and a metal semiconductor field effect transistor (MESFET), each of which has a heterojunction for generating a two-dimensional carrier gas layer that serves as a conduction path of carriers, generally has normally-on characteristics. In order to turn off the HEMT, the MESFET or the like, which has the normally-off characteristics, a negative power supply for setting a gate electrode thereof at a negative potential is required, and an electric circuit is necessarily increased in price. Accordingly, a semiconductor device having normally-off characteristics is required (for example, refer to Japanese Patent Laid-Open Publication No. 2007-19309).
In order to form the semiconductor device having the normally-off characteristics, a special gate structure is frequently adopted. As this special gate structure for imparting the normally-off characteristics to the semiconductor device concerned, a stacked gate structure composed of metal, an insulating material, a semiconductor and the like is known in addition to a recess gate structure as contrivance of a shape to thin an electron supply layer (for example, refer to Japanese Patent Laid-Open Publication No. 2006-32650, Pamphlet of International Publication No. 2003/71607 and Japanese Patent Laid-Open Publication No. 2005-244072). Owing to process restrictions, these gate structures are fabricated by using a technology such as thermal evaporation, electron beam (EB) evaporation, sputtering and chemical vapor deposition (CVD), which are capable of performing deposition at a relatively low temperature.
However, in the above-described semiconductor device having the special gate structure for imparting the normally-off characteristics thereto, in the case of targeting the semiconductor device concerned to a power device to be used at a high voltage, there has been a problem that the gate structure may be highly possibly broken as a result of being incapable of withstanding the high voltage, and that reliability of the semiconductor device is thereby low.